Bipolar disorder is a disabling mental illness characterized by episodes of both elevated or irritable mood and depression. See, e.g., Müller-Oerlinghausen et al., Lancet 2002; 359:241-7; and Frye M A., N Engl J Med 2011; 364:51-9. Currently, lithium is the first-line choice for maintenance treatment in bipolar disorder and reduces risk of relapse and suicide. See, e.g., Fountoulakis et al., Eur Arch Psychiatry Clin Neurosci 2012; 262 Suppl 1:1-48. However, many patients do not respond to lithium treatment.